The present invention relates to a method for fabricating semiconductor devices and, more particularly, a method for fabricating a TFT (Thin Film Transistor) using crystalline silicon film as an active region.
TFTs using crystalline silicon film as an active region have been widely used for active matrix type liquid crystal devices, contact type image sensors and the like. Further, in recent years, these devices have been increasingly growing toward higher resolutions, and what is more, there has been made an attempt to build in, on the same board, not only driver circuits but also IC (Integrated Circuit) functions of control circuits, data processing circuits and the like that have been externally provided in the prior arts. For these reasons, TFTs are demanded to attain further enhancement in performance (higher-speed operations, lower leak current and lower-voltage operations).
Also, by enhancing the TFT performance up to an equivalency to MOS (Metal Oxide Semiconductor) transistors of single crystal silicon, it becomes possible to achieve newly functioned devices and so-called 3D ICs, which take advantage of characteristics as SOI (Silicon On Insulator). Like this, for enhancement in TFT performance, it is indispensable to achieve higher qualities of crystalline silicon films constituting the active region, i.e., scale-up of crystal grains, improvement in orientation characteristics, reduction in defect density and reduction in impurities.
Conventionally, as a method for achieving higher qualities of crystalline silicon films, there has been used a method in which a metallic element serving for acceleration of crystallization is introduced into an amorphous silicon film formed on an insulating substrate so that amorphous silicon is crystallized, and thereafter the metallic element is removed or reduced by gettering process. Such methods for achieving higher qualities of crystalline silicon films are disclosed in, for example, Japanese Patent Laid-Open Publications HEI 7-192998 and HEI 10-223533. These are described below in detail and separately.
(A) Japanese Patent Laid-Open Publication HEI 7-192998
After a metallic element serving for acceleration of crystallization is introduced into an amorphous silicon film so that the amorphous silicon film is crystallized, the amorphous silicon film is heated in an oxidative atmosphere and thereby oxidized, by which the metallic element is gettered into the oxide. More specifically, this method is carried out as follows (see FIGS. 6A to 6F):
(1) As shown in FIG. 6A, on an insulating substrate (glass substrate) 1, oxide as a base coat film 2 is deposited to a thickness of 2000 xc3x85 by sputtering process, and then amorphous silicon 3 is deposited to a thickness of about 1000 xc3x85.
(2) As shown in FIG. 6B, on the amorphous silicon 3, oxide 4 is deposited to a thickness of 1000 xc3x85 or more by sputtering process, and patterning is performed, by which a mask is formed.
(3) A metallic element of Ni element 6 or the like is introduced into the amorphous silicon 3 at an opening portion 5.
(4) As shown in FIG. 6C, the oxide (mask layer) 4 on the amorphous silicon 3 is removed by etching process, and heated at 550xc2x0 C., so that surrounding amorphous silicon 3 is crystallized out of a Ni-element high concentration region 7, by which a crystalline silicon film 8 is obtained.
(5) The crystalline silicon film 8 is patterned into an island shape as shown in FIG. 6D, and the surface of the crystalline silicon film 8 is oxidized as shown in FIG. 6E to thereby form oxide 10. In this way, by incorporating the Ni element into the oxide 10, Ni is gettered into the oxide 10. As a result, the concentration of Ni in the crystalline silicon film 8 is reduced. It is noted that reference numeral 9 denotes a crystal growth end.
(6) Finally, as shown in FIG. 6F, the surface oxide 10 is completely removed.
(B) Japanese Patent Laid-Open Publications HEI 10-223533
After a metallic element serving for acceleration of crystallization is introduced into an amorphous silicon film so that the amorphous silicon film is crystallized, a mask is formed selectively on the amorphous silicon film. Then, one kind or a plurality of kinds of elements among nitrogen, phosphorus, arsenic, antimony and bismuth are added, and the metal in regions where the element or elements are not added is gettered into the region where the element or elements are added. More specifically, this is carried out as follows (see FIGS. 7A to 7F):
(1) As shown in FIG. 7A, on a glass substrate 11 with a 2000 xc3x85 silicon oxide (not shown) formed thereon, 500 xc3x85 amorphous silicon film 12 is deposited by plasma CVD (chemical vapor deposition) process.
(2) Further, Ni acetate 13 is formed by spin-coating a Ni acetate solution having a Ni concentration of 100 ppm, and a metal of Ni is introduced into the surface of the amorphous silicon film 12.
(3) A 4 hour heating process is performed at a temperature of 600xc2x0 C. so that the amorphous silicon film 12 is crystallized, by which a crystalline silicon film 14 is obtained as shown in FIG. 7B.
(4) With KrF (krypton fluoride) excimer laser beam (wavelength: 248 nm) applied, a laser annealing process is performed.
(5) As shown in FIG. 7C, 1000 xc3x85 silicon nitride 15 is deposited on the crystalline silicon film 14 by plasma CVD process.
(6) By etching the silicon nitride 15, as shown in FIG. 7D, a mask 16 for implantation of phosphorus is formed.
(7) By plasma doping process, phosphorus is implanted at a dose of 5xc3x971014 atom/cm2.
(8) As shown in FIG. 7E, a 2 hour heating process is performed at a temperature of 600xc2x0 C. in a nitrogen atmosphere, by which Ni of a crystalline silicon film 18 under the mask 16 is moved in the arrow direction and thereby gettered into a region 17 where phosphorus has been implanted. As a result, the concentration of Ni in the crystalline silicon film 18, in which phosphorus has not been implanted, is reduced.
(9) As shown in FIG. 7F, the region 17, in which phosphorus has been implanted by using the mask 16 (to which Ni has moved), is removed, by which the mask 16 is removed. Finally, peripheral part 19 of the gettered region 18 is removed by using a mask (not shown) smaller than the mask 16, and the used mask is also removed.
However, the conventional method including the gettering of a metallic element has the following problems:
(1) Japanese Patent Laid-Open Publication HEI 7-192998
In the growth of a crystalline silicon film using a metallic element serving for acceleration of crystallization, since enough amount of a metallic element is necessitated before crystallization in order to obtain enough crystalline growth, a large amount of the metallic element is contained in the film just before the crystallization. However, after the crystallization, it is desirable that the concentration of the metallic element be as low as possible in terms of the quality of crystalline silicon film and TFT characteristics. That is, the remaining metallic element serves as impurities that adversely affect the TFT characteristics, so that high mobility cannot be obtained and the resulting leak current is large.
Unfortunately, in the crystalline silicon film that has been crystallized by heating process performed after the implementation of a metal serving for acceleration of crystallization into amorphous silicon film, the metal serving for acceleration of crystallization is distributed not uniformly but unevenly. In particular, at grain boundaries at which a plurality of crystal grains contact one another, high-concentration metals are present in a state of compound with silicon. In Japanese Patent Laid-Open Publication HEI 7-192998, the crystalline silicon film is oxidized in order to reduce the remaining metals, in which case oxidation considerably proceeds at grain boundaries where metals are present at high concentrations, and after the oxidation, irregularities of the crystalline silicon film surface are considerably increased. There are some cases where pinholes may be formed particularly at grain boundaries where a plurality of crystal grains contact one another. Avoiding these possibilities makes it impossible to attain enough oxidation for the purpose of gettering, as a problem. Also, irregularities of the crystalline silicon film surface adversely affect the TFT characteristics and cause the carrier scattering to be so large that successful characteristics could not be obtained, as another problem.
Still another problem of Japanese Patent Laid-Open Publication HEI 7-192998 lies in the lowness of gettering capability. Below are shown results of an experiment conducted by the present inventor. This experiment was conducted through the following steps:
(a) Depositing an amorphous silicon film to a film thickness of 650 xc3x85 on a quartz substrate by low pressure CVD process;
(b) Applying a Ni acetate solution having a Ni concentration of 10 ppm by spin coating process so that Ni is introduced to the amorphous silicon film surface at a concentration of 1xc3x971013 atom/cm2;
(c) Conducting a 12 hour heating process at a temperature of 600xc2x0 C. in a nitrogen atmosphere, thereby crystallizing the amorphous silicon film to obtain a crystalline silicon film;
(d) Forming 500 xc3x85 thick oxide on the crystalline silicon film by oxidizing the crystalline silicon film at a temperature of 950xc2x0 C. in an O2 atmosphere with HCl added thereto, and incorporating Ni into this oxide; and
(e) Analyzing Ni in the crystalline silicon film and the oxide by ICP-MS (Inductively Coupled Plasma Mass Spectrometry) process.
As a result of this analysis, the resulting Ni concentration in the crystalline silicon film was 1.0xc3x971017 atom/cm2, which is too high to obtain successful TFT characteristics, making it found that the gettering was poor. It was also found by surface observation that pinholes were present in the region where a plurality of crystal grains contact one another.
(2) Japanese Patent Laid-Open Publication HEI 10-223533
An issue of Japanese Patent Laid-Open Publication HEI 10-223533 lies in a lowness of its gettering capability. Below are shown results of an experiment conducted by the present inventor. This experiment was conducted through the following steps:
(a) Depositing an amorphous silicon film to a film thickness of 650 xc3x85 on a quartz substrate by low pressure CVD process;
(b) Applying a Ni acetate solution having a Ni concentration of 10 ppm by spin coating process so that Ni is introduced to the amorphous silicon film surface at a concentration of 1xc3x971013 atom/cm2;
(c) Conducting a 12 hour heating process at a temperature of 600xc2x0 C. in a nitrogen atmosphere, thereby crystallizing the amorphous silicon film to obtain a crystalline silicon film;
(d) Depositing 2000 xc3x85 silicon oxide on the crystalline silicon film by atmospheric pressure CVD process;
(e) Forming a mask by etching the silicon oxide;
(f) Implanting phosphorus at a concentration of 4xc3x971015 atom/cm2 by ion implementation;
(g) Moving and gettering Ni into the region where the phosphorus has been implanted by conducting a 12 hour heating process at a temperature of 600xc2x0 C. in a nitrogen atmosphere;
(h) With the silicon oxide used as an etching mask, etching and removing the region where Ni has been gettered; and
(i) After removing the silicon oxide, analyzing the Ni in the silicon oxide by ICP-MS process.
As a result of this analysis, the resulting Ni concentration in the crystalline silicon film was 1.8xc3x971017 atom/cm2, which is too high to obtain successful TFT characteristics, making it found that the gettering was poor. This could be attributed to the fact that this metallic element gettering process, in principle, involves diffusing Ni horizontally (laterally) over long distances in the silicon oxide.
Therefore, an object of the present invention is to provide a method for fabricating a semiconductor device using crystalline silicon film which is small in surface irregularities and so smooth, free from pinholes and high in quality, and which is obtained by performing enough gettering of metal to obtain successful crystallinity and successful TFT characteristics.
In order to achieve the above object, the present invention provides a method for fabricating a semiconductor device comprising: a step for crystallizing an amorphous silicon film to obtain a crystalline silicon film by introducing a metallic element serving for acceleration of crystallization onto the amorphous silicon film, and by performing a first heating process in an non-oxidative atmosphere; a first gettering step for removing or reducing the metallic element present in at least a partial region of the crystalline silicon film by performing a second heating process in a non-oxidative atmosphere; a second gettering step for further removing or reducing the metallic element present in the region of the crystalline silicon film where the first gettering has been effected, by performing a third heating process in an oxidative atmosphere; and a step for removing oxide formed by the second gettering step.
With this constitution, during the vertical growth of the crystalline silicon film, the first gettering process without involving oxidation is performed by performing the second heating process in a non-oxidative atmosphere, and thereafter the second gettering process involving oxidation is performed. Therefore, since the concentration of the metallic element is reduced by the first gettering process to such a level that oxidation does not cause increase of irregularities or occurrence of pinholes, a high-quality crystalline silicon film free from surface irregularities and pinholes can be obtained. Also, in the second gettering process, enough oxidation can be effected without minding any increase of irregularities and occurrence of pinholes, so that the concentration of the metallic element can be reduced to an extremely low level.
Accordingly, a semiconductor device is fabricated with the high-quality crystalline silicon film used as an active layer, so that a high-performance semiconductor device capable of high-speed operation, low-leak current and low-voltage operation can be obtained.
In one embodiment of the present invention, the metallic element serving for acceleration of crystallization is one kind of metallic element or a plurality of metallic elements selected from among iron, cobalt, nickel, copper, ruthenium, rhodium, palladium, osmium, iridium, platinum and gold.
With this constitution, the crystallization of the amorphous silicon film is accelerated by the action of the metallic element, so that the crystalline silicon film can be formed more efficiently, as compared with the case only by thermal annealing.
In one embodiment of the present invention, in the first gettering step, one kind of element or a plurality of kinds of elements selected from among nitrogen, phosphorus, arsenic, antimony and bismuth are introduced onto a surface or into film of a region of the crystalline silicon film except the region from which the metallic element is to be removed or reduced, and thereafter the second gettering process is performed in a non-oxidative atmosphere containing an inert gas such as nitrogen, hydrogen, argon and helium, whereby the metallic element is gettered into the element-introductory region.
With this constitution, since an element which is easily combinable with the metallic element serving for acceleration of crystallization is introduced into the crystalline silicon film, the metallic element can be gettered efficiently even by the first gettering process involving no oxidation in the non-oxidative atmosphere.
In one embodiment of the present invention, in the first gettering step, oxide is deposited on the surface of the crystalline silicon film, the metallic element is gettered into the oxide by performing the second heating process in a non-oxidative atmosphere containing an inert gas such as nitrogen, hydrogen, argon and helium, and thereafter the oxide is removed.
With this constitution, since the metallic element is gettered into the oxide deposited on the surface of the crystalline silicon film, there is no need of patterning for introducing an element which is easily combinable with the metallic element into the crystalline silicon film, as is necessary in an embodiment of the invention. Therefore, the first gettering process is carried out more simply, as compared with the case of the embodiment.
In one embodiment of the present invention, in the first gettering step, oxide is deposited on the surface of the crystalline silicon film, the metallic element is gettered into the oxide by performing the second heating process in a non-oxidative atmosphere containing at least one kind of halogen element selected from among hydrogen chloride, hydrogen fluoride, hydrogen bromide, chlorine, fluorine and bromine, and thereafter the oxide is removed.
With this constitution, the first gettering process is performed in a non-oxidative atmosphere containing a halogen element which is easily combinable with the metallic element serving for acceleration of crystallization. Thus, the gettering effect can be enhanced.
In one embodiment of the present invention, in the second gettering step, by the third heating process in an oxidative atmosphere, thermal oxide is formed on the surface of the crystalline silicon film and the metallic element present in the region of the crystalline silicon film where the first gettering has been effected is incorporated into the thermal oxide, whereby the metallic element is further gettered.
With this constitution, by the third heating process in an oxidative atmosphere, the metallic element is gettered into the thermal oxide formed on the surface of the crystalline silicon film. Therefore, the step of forming oxide for gettering the metallic element is not particularly required, and the semiconductor device fabrication process is simplified.
In one embodiment of the present invention, in the second gettering step, oxide is deposited on the surface of the crystalline silicon film, and by the third heating process in an oxidative atmosphere, the metallic element present in the region of the crystalline silicon film where the first gettering has been effected is incorporated into the oxide, whereby the metallic element is further gettered.
With this constitution, after the oxide for gettering the metallic element is formed on the surface of the crystalline silicon film, the metallic element is gettered by the third heating process in an oxidative atmosphere. Thus, the oxidation of the crystalline silicon film by the third heating process is performed via a previously formed oxide, and a higher-quality crystalline silicon film which is smaller in surface irregularities and freer from pinholes can be obtained as compared with such a case where the oxidation by the third heating process is performed directly on the crystalline silicon film as in an embodiment of the invention.
In one embodiment of the present invention, the oxidative atmosphere in which the third heating process is performed contains at least one kind of halogen element selected from among hydrogen chloride, hydrogen fluoride, hydrogen bromide, chlorine, fluorine and bromine.
With this constitution, the second gettering process is performed in an oxidative atmosphere containing a halogen element which is easily combinable with the metallic element serving for acceleration of crystallization. Thus, the gettering effect can be enhanced.
In one embodiment of the present invention, temperature for the third heating process is 700xc2x0 C. or higher and 1150xc2x0 C. or lower.
With this constitution, the third heating process in the second gettering process is performed at a high temperature not lower than 700xc2x0 C. and not higher than 1150xc2x0 C. Thus, the diffusion of the metallic element in the oxide is accelerated, so that the gettering effect can be enhanced.
The present invention also provides a method for fabricating a semiconductor device comprising: a step for crystallizing an amorphous silicon film to obtain a crystalline silicon film by introducing a metallic element serving for acceleration of crystallization onto a partial region of the amorphous silicon film, and by performing a first heating process in an non-oxidative atmosphere; a first gettering step for removing or reducing the metallic element present in a region of the crystalline silicon film except regions of the crystalline silicon film vertically crystallized from the partial region, by performing a second heating process in a non-oxidative atmosphere; a second gettering step for further removing or reducing the metallic element present in the region of the crystalline silicon film where the first gettering has been effected, by performing a third heating process in an oxidative atmosphere; and a step for removing oxide formed by the second gettering step.
With this constitution, during the horizontal growth of the crystalline silicon film, the first gettering process without involving oxidation is performed by performing the second heating process in a non-oxidative atmosphere, and thereafter the second gettering process involving oxidation is performed. Therefore, since the concentration of the metallic element is reduced by the first gettering process to such a level that oxidation does not cause increase of irregularities or occurrence of pinholes, a high-quality crystalline silicon film free from surface irregularities and pinholes can be obtained. Also, in the second gettering process, enough oxidation can be effected without minding any increase of irregularities and occurrence of pinholes, so that the concentration of the metallic element can be reduced to an extremely low level.
Accordingly, a semiconductor device is fabricated with the high-quality crystalline silicon film used as an active layer, so that a high-performance semiconductor device capable of high-speed operation, low-leak current and low-voltage operation can be obtained.
In one embodiment of the present invention, the metallic element serving for acceleration of crystallization is one kind of metallic element or a plurality of metallic elements selected from among iron, cobalt, nickel, copper, ruthenium, rhodium, palladium, osmium, iridium, platinum and gold.
With this constitution, the crystallization of the amorphous silicon film is accelerated by the action of the metallic element, so that the crystalline silicon film can be formed more efficiently, as compared with the case only by thermal annealing.
In one embodiment of the present invention, in the first gettering step, one kind of element or a plurality of kinds of elements selected from among nitrogen, phosphorus, arsenic, antimony and bismuth are introduced onto a surface or into film of a region of the crystalline silicon film including the region into which the metallic element has been introduced, and thereafter the second heating process is performed in a non-oxidative atmosphere containing an inert gas such as nitrogen, hydrogen, argon and helium, whereby the metallic element is gettered into the element-introductory region.
With this constitution, since an element which is easily combinable with the metallic element serving for acceleration of crystallization is introduced into the crystalline silicon film, the metallic element can be gettered efficiently even by the first gettering process involving no oxidation in the non-oxidative atmosphere.
In one embodiment of the present invention, in the first gettering step, oxide is deposited on the surface of the crystalline silicon film, the metallic element is gettered into the oxide by performing the second heating process in a non-oxidative atmosphere containing an inert gas such as nitrogen, hydrogen, argon and helium, and thereafter the oxide is removed.
With this constitution, since the metallic element is gettered into the oxide deposited on the surface of the crystalline silicon film, there is no need of patterning for introducing an element which is easily combinable with the metallic element into the crystalline silicon film, as is necessary in an embodiment of the invention. Therefore, the first gettering process is carried out more simply, as compared with the case of the embodiment.
In one embodiment of the present invention, in the first gettering step, oxide is deposited on the surface of the crystalline silicon film, the metallic element is gettered into the oxide by performing the second heating process in a non-oxidative atmosphere containing at least one kind of halogen element selected from among hydrogen chloride, hydrogen fluoride, hydrogen bromide, chlorine, fluorine and bromine, and thereafter the oxide is removed.
With this constitution, the first gettering process is performed in a non-oxidative atmosphere containing a halogen element which is easily combinable with the metallic element serving for acceleration of crystallization. Thus, the gettering effect can be enhanced.
In one embodiment of the present invention, in the second gettering step, by the third heating process in an oxidative atmosphere, thermal oxide is formed on the surface of the crystalline silicon film and the metallic element present in the region of the crystalline silicon film where the first gettering has been effected is incorporated into the thermal oxide, whereby the metallic element is further gettered.
With this constitution, by the third heating process in an oxidative atmosphere, the metallic element is gettered into the thermal oxide formed on the surface of the crystalline silicon film. Therefore, the step of forming oxide for gettering the metallic element is not particularly required, and the semiconductor device fabrication process is simplified.
In one embodiment of the present invention, in the second gettering step, oxide is deposited on the surface of the crystalline silicon film, and by the third heating process in an oxidative atmosphere, the metallic element present in the region of the crystalline silicon film where the first gettering has been effected is incorporated into the oxide, whereby the metallic element is further gettered.
With this constitution, after the oxide for gettering the metallic element is formed on the surface of the crystalline silicon film, the metallic element is gettered by the third heating process in an oxidative atmosphere. Thus, the oxidation of the crystalline silicon film by the third heating process is performed via a previously formed oxide, and a higher-quality crystalline silicon film which is smaller in surface irregularities and freer from pinholes can be obtained as compared with such a case where the oxidation by the third heating process is performed directly on the crystalline silicon film as in an embodiment of the invention.
In one embodiment of the present invention, the oxidative atmosphere in which the third heating process is performed contains at least one kind of halogen element selected from among hydrogen chloride, hydrogen fluoride, hydrogen bromide, chlorine, fluorine and bromine.
With this constitution, the second gettering process is performed in an oxidative atmosphere containing a halogen element which is easily combinable with the metallic element serving for acceleration of crystallization. Thus, the gettering effect can be enhanced.
In one embodiment of the present invention, temperature for the third heating process is 700xc2x0 C. or higher and 1150xc2x0 C. or lower.
With this constitution, the third heating process in the second gettering process is performed at a high temperature not lower than 700xc2x0 C. and not higher than 1150xc2x0 C. Thus, the diffusion of the metallic element in the oxide is accelerated, so that the gettering effect can be enhanced.